This is for learning purpose. This will help engineering students to learn Operating System in details.

1. Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon-423 603
(An Autonomous Institute, Affiliated to Savitribai Phule Pune University, Pune)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 Certified
Department of Computer Engineering
(NBA Accredited)
Prof. A. V. Brahmane
Assistant Professor
E-mail : brahmaneanilkumarcomp@sanjivani.org.in
Contact No: 91301 91301 Ext :145, 9922827812
Subject- Operating System and Administration (CO2013)
Unit II- Buffer Cache

2. Content
• Buffer cache
• Buffer Headers
• Structre of the Buffer Pool
• Scenaios for retrival of a buffer
• Reading and Writing Disk blocks
• Advntages and disadvantages of buffer cache
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon

3. The Buffer Cache
• The kernel could read and write directly to and from the disk for all the file system accesses, but
system response time and throughput will be poor because of the slow disk transfer rate.
• The kernel therefore attempts to minimize the frequency of disk access by keeping a pool of data
buffers, called the buffer cache, which contains data in recently used disk blocks.
• Architecturally, it is positioned between file subsystem and device drivers.
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon

4. Buffer Headers
• During system initialization, the kernel allocates space for a number of buffers,
configurable according to memory size and performance constraints.
• Two parts of the buffer:
1.a memory array that contains data from the disk.
2.buffer header that identifies the buffer.
• Data in a buffer corresponds to data in a logical disk block on a file system. A disk
block can never map into more than one buffer at a time.
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5. Buffer Header
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6. • The device number fields specifies the logical file system (not physical device) and block
number block number of the data on disk. These two numbers uniquely identify the buffer. The
status field summarizes the current status of the buffer. The ptr to data area is a pointer to the
data area, whose size must be at least as big as the size of a disk block.
• The status of a buffer is a combination of the following conditions:
• Buffer is locked / busy
• Buffer contains valid data
• Kernel must write the buffer contents to disk before reassigning the buffer; called as "delayed-
write"
• Kernel is currently reading or writing the contexts of the buffer to disk
• A process is currently waiting for the buffer to become free.
• The two set of pointers in the header are used for traversal of the buffer queues (doubly linked
circular lists).
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon

7. Structure of the Buffer Pool
• The kernel follows the least recently unused (LRU) algorithm for the buffer pool.
• The kernel maintains a free list of buffers that preserves the least recently used order.
• Dummy buffer header marks the beginning and end of the list.
• All the buffers are put on the free list when the system is booted.
• When the kernel wants any buffer, it takes it from the head of the free list.
• But it can also take a specific buffer from the list.
• The used buffers, when become free, are attached to the end of the list, hence the buffers closer
and closer to the head of the list are the most recently used ones.
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon

8. Free list of buffers
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9. DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon
• When the kernel accesses a disk block, it searches for the buffer with the
appropriate device-block number combination.
• Rather than search the entire buffer pool, it organizes the buffers into separate
queues, hashed as a function of the device and block number.
• The hash queues are also doubly linked circular lists.
• A hashing function which uniformly distributes the buffers across the lists is used.
• But it also has to be simple so that the performance does not suffer.

10. Buffers on the Hash Queues
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon

11. Buffers on the Hash Queues
• The hash function shown in the figure only depends on the block number; real
hash functions depend on device number as well.
• Every disk block in the buffer pool exists on one and only one hash queue and
only once on that queue.
• However, presence of a buffer on a hash queue does not mean that it is busy, it
could well be on the free list as well if its status is free.
• Therefore, if the kernel wants a particular buffer, it will search it on the queue.
But if it wants any buffer, it removes a buffer from the free list. A buffer is always
on a hash queue, but it may or may not be on the free list
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon

12. Scenarios for Retrieval of a Buffer
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon
• The algorithms for reading and writing disk blocks use the algorithm getblk to allocate buffers
from the pool. There are 5 typical scenarios the kernel may follow in getblk to allocate a buffer for
a disk block.
• 1.Block is found on its hash queue and its buffer is free.
• 2.Block could not be found on the hash queue, so a buffer from the free list is allocated.
• 3.Block could not be found on the hash queue, and when allocating a buffer from free list, a
buffer marked "delayed write" is allocated. Then the kernel must write the "delayed write" buffer
to disk and allocate another buffer.
• 4.Block could not be found on the hash queue and the free list of buffers is empty.
• 5.Block was found on the hash queue, but its buffer is currently busy.

13. Algorithm for Buffer allocation
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon

14. Algorithm for releasing a buffer
• When using the buffer, the kernel always marks the buffer as busy so that no
other process can access it.
• When the kernel finishes using the buffer, it releases the buffer according to the
algorithm brelse
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon

15. Buffer releasing.....
• Buffer contents are old only if it is marked as "delayed write", in that case and in the case where
the data is not valid (for example, due to I/O corruption), the buffer is put in the beginning of the
free list as its data is not valid or old.
• Otherwise the data is valid as the buffer is put at the end to follow the LRU strategy.
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon

16. Scenario 1
• The states of hash queues for different scenarios are shown in following figures :
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17. Scenario 2
• Here the buffer is not on the hash queue, so a buffer from free list is removed and then its device
and block numbers are changed.
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon

18. Scenario 3
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• Cannot find block on the hash queue => allocate a buffer from free list but buffer
on the free list marked “delayed write” => flush “delayed write” buffer and
allocate another buffer.

19. Scenario 4
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Cannot find block on the hash queue and free list of buffer also empty.

20. Scenario 5
• Block in the hash queue, but buffer is busy.
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21. Reading and Writing Disk Blocks
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22. Reading and writing disk blocks....
•
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23. Writing a disk block
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24. Advantages of the buffer cache
• Uniform disk access => system design simpler
• Copying data from user buffers to system buffers => eliminates the need for
special alignment of user buffers.
• Use of the buffer cache can reduce the amount of disk traffic.
• Single image of of disk blocks contained in the cache => helps insure file system
integrity
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon

25. Disadvantages of the buffer cache
DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon
• Delayed write => vulnerable to crashes that leave disk data in
incorrect state
• An extra data copy when reading and writing to and from user
processes => slow down when transmitting large data
•

26. • Thank you …
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